A large-sized conveyance vehicle called a dump truck is provided with a loading platform provided on a vehicle body capable of moving up/down. This dump truck loads an objects to be transported (a load) such as crushed stones, earth and sand and the like on the loading platform and transports and conveys it to an unloading site and a load collecting site such as an export port or the like, for example.
The conveyance vehicle is composed of an automotive vehicle body, a loading platform provided on a vehicle body for loading an objects to be transported such that the front side is rotatable in an upward and downward direction by using the rear side as a fulcrum, a hoist cylinder provided capable of extension/contraction between the loading platform and the vehicle body and rotationally moving the loading platform upward by extending, a hydraulic power source for supplying a pressurized oil for operation to the hoist cylinder, and a control valve device provided between the hydraulic power source and the hoist cylinder and controlling supply and discharge of the pressurized oil with respect to the hoist cylinder.
The control valve device used in the conveyance vehicle has four switching positions in total, which are a neutral position, a raising position, a lowering position, and a floating position. The control valve device stops supply or discharge of the pressurized oil with respect to the hoist cylinder at the neutral position so as to stop movement of the hoist cylinder, while at the raising position, the control valve device extends the hoist cylinder by supply or discharge of the pressurized oil so as to rotationally move the loading platform upward. Moreover, the control valve device contracts the hoist cylinder by supply or discharge of the pressurized oil at the lowering position so as to rotationally move the loading platform downward and contracts the hoist cylinder by discharging the pressurized oil by the weight of the loading platform at the floating position so as to allow free fall of the loading platform.
This control valve device is selectively switched to one of the four switching positions in total by operation of a operating lever by an operator. Therefore, the conveyance vehicle runs to the load collecting site in a state in which an objects to be transported such as earth and sand is loaded on the loading platform and then, extends a rod of the hoist cylinder so as to rotationally move the front side of the loading platform upward and discharges the loaded objects to be transported to the load collecting site along inclination of this loading platform (Patent Document 1).